The FBI guards its high-tech secrets so carefully that officials once warned agents not to share details even with federal prosecutors for fear they might eventually go on to work as defense attorneys, newly disclosed records show.

A supervisor also cautioned the bureau’s “technically trained agents” in a 2003 memo not to reveal techniques for secretly entering and bugging a suspect’s home to other agents who might be forced to reveal them in court. “We need to protect how our equipment is concealed,” the unnamed supervisor wrote.